Risky business for a juvenile marine predator? Testing the influence of foraging strategies on size and growth rate under natural conditions
Abstract
Mechanisms driving selection of body size and growth rate in wild marine
vertebrates are poorly understood, thus limiting knowledge of their fitness
costs at ecological, physiological and genetic scales. Here, we indirectly
tested whether selection for size-related traits of juvenile sharks that inhabit
a nursery hosting two dichotomous habitats, protected mangroves (low
predation risk) and exposed seagrass beds (high predation risk), is influenced
by their foraging behaviour. Juvenile sharks displayed a continuum
of foraging strategies between mangrove and seagrass areas, with some
individuals preferentially feeding in one habitat over another. Foraging
habitat was correlated with growth rate, whereby slower growing, smaller
individuals fed predominantly in sheltered mangroves, whereas larger,
faster growing animals fed over exposed seagrass. Concomitantly, tracked
juveniles undertook variable movement behaviours across both the low
and high predation risk habitat. These data provide supporting evidence
for the hypothesis that directional selection favouring smaller size and
slower growth rate, both heritable traits in this shark population, may be
driven by variability in foraging behaviour and predation risk. Such evolutionary
pathways may be critical to adaptation within predator-driven
marine ecosystems.